Expose the rest of LoadedFile in headers.

This will allow it to be used more conveniently for things other than
key files.

For the moment, the implementation still lives in sshpubk.c. Moving it
out into utils.c or misc.c would be nicer, but it has awkward
dependencies on marshal.c and the per-platform f_open function.
Perhaps another time.

misc.h

@@ -406,4 +406,25 @@ static inline char *stripctrl_string(StripCtrlChars *sccpub, const char *str)
     return stripctrl_string_ptrlen(sccpub, ptrlen_from_asciz(str));
 }
 
+/*
+ * A mechanism for loading a file from disk into a memory buffer where
+ * it can be picked apart as a BinarySource.
+ */
+struct LoadedFile {
+    char *data;
+    size_t len, max_size;
+    BinarySource_IMPLEMENTATION;
+};
+typedef enum {
+    LF_OK,      /* file loaded successfully */
+    LF_TOO_BIG, /* file didn't fit in buffer */
+    LF_ERROR,   /* error from stdio layer */
+} LoadFileStatus;
+LoadedFile *lf_new(size_t max_size);
+void lf_free(LoadedFile *lf);
+LoadFileStatus lf_load_fp(LoadedFile *lf, FILE *fp);
+LoadFileStatus lf_load(LoadedFile *lf, const Filename *filename);
+static inline ptrlen ptrlen_from_lf(LoadedFile *lf)
+{ return make_ptrlen(lf->data, lf->len); }
+
 #endif